Proton exchange membrane (PEM) fuel cells are fast approaching commercialization for application in the transportation, military and stationary sectors. One of the needs for the operation of a PEM fuel cell is that of high purity hydrogen with less than ˜10 ppm of impurities such as CO. It is therefore expected that if PEM fuel cells are to have wide applications, an infrastructure to produce tonnage quantities of high purity hydrogen has to be available.
Presently tonnage hydrogen is produced by reformation of hydrocarbon fuel followed by the water gas shift reaction and pressure wing adsorption (PSA) of the CO2. The major disadvantage of the current process especially in reference to PEM fuel cell application is that the purity levels that can be attained are low (80-90%). Impurities such as CO and CH4 remain in the hydrogen which can poison the anodes of the PEM fuel cell.
Operation of an electrochemical reactor for production of hydrogen from synthesis gas or other reformate gases based on the membrane having mixed oxygen ion and electronic conductivity has been described. See, commonly owned International Published Application WO 03/089117, which is incorporated in its entirety by reference. The process, generally conducted at temperatures of approximately 800-1000° C., involves the use of a cell in which a mixture of reformate gas and steam are flowed on one side of a dense solid state ceramic membrane, while steam is passed on the other side. High purity hydrogen is generated on the steam side. The ceramic membrane is a mixed electronic and ionic conductivity (MEIC) membrane.
MIEC membranes have been used in oxygen separation including partial oxidation of methane (POX), oxidative coupling of alkanes to form alkenes, and oxygen separation from air for medical applications. Gas separation processes using MIEC membranes require membranes with high chemical stability and high ambipolar conductivity, i.e., applying equally to positive and negative charges. Complex oxide perovskites, La1−xSrxCoyFe1−yO3-δ (LSCF) and La1−xCaxFeO3−δ (LCF), have high ambipolar conductivities and oxygen surface exchange coefficients, and are excellent candidate materials as oxygen separation membranes. However, most of the targeted membrane separation applications for LSCF and LCF involve relatively high oxygen partial pressure (10−4 to 1 atm). Under such conditions these perovskites are quite stable. LSCF and LCF are not expected to retain their chemical and structural stability at lower oxygen partial pressures.
An important requirement for MIEC gas separation membranes is that they remain chemically and structurally stable under the operating conditions that exist on both sides of the membrane in gas separation processes. Recent work on LSCF and LCF systems demonstrate that LSCF and LCF perovskites do not possess the requisite chemical stability to function as membrane separators in all processes and in particular in hydrogen separation processes including reformate gases where the oxygen partial pressure can be quite low.